Electromagnetic focusing coil



May 28, 1940. i D. v. RIDQEWAY y2,202,505

ELECTHOMAGNETIC FOCUSING COIL AFiled June 27, 1939 FIG. I

TTOPNEY Patented May 28, 1940 UNITED STATES PATENT OFFICE ELECTROMAGNETIC FOCUSING COIL Application June 27,

1939, Serial N0. 281,372

In Great Britain January 13, 1939 3 Claims.

This invention relates to improvements in electromagnetic focus coils for use with electron discharge devices and is especially concerned with an improved method of screening and mounting a focusing coil such as is used for focusing the electron beam in a cathode ray tube.

It has previously been proposed to screen a short-multi-layer focus coil by placing it in an iron shielding member comprising two annular end pieces and a cylindrical member connecting them. It is found that when the focus coil is placed closely adjacent to an unsymmetrical electromagnetic deflecting yoke, distortion of the focusing field extending outside the vscreening member is produced owing to the asymmetry of the said yoke and it is an object of the present invention to provide a focus coil in which the spread of the focusing field may be substantially reduced.

According to the present invention there is provided a focus coil for use with an electron discharge device comprising an electromagnetic coil enclosed by a screening member or' members so arranged that at least two annular shielding members spaced from each other `by a distance substantially greater than their thickness are interposed immediately adjacent to each end of said coil.

A screening member according to the pr'esent invention may conveniently be constructed by enclosing the coil in a screening member composed of a number of pressings of ferro-magnetic material, the said pressings being such that they have an annular end portion, a cylindrical portion and* a flange attached to the end of said cylindrical portion other than that to whi-ch said annular portion is attached. Pairs of such pressings may be assembled together to form a screening member according to the present invention.

In order that the present invention may be more particularly described reference is now made to the accompanying diagrammatic drawing of which:

Figure 1 is a cross-section through a focus coil and screening member according to the present invention, while Figure 2 illustrates a convenient method of mounting a screen focus coil according to the invention.

In Figure 1 a short multi-layer focus coil I is enclosed in a screening member built up of pairs 2, 3 and 4, 5 of pressings of the type described above. may be fastened together by suitable bolts or rivets passed through holes 6 to form a single structural unit. Obviously another pair of more The whole screening assemblyl (Ol. Z50-161) deeply dished pr'essings may be addedif still further screening is desired.

A convenient method of mounting a focus coil as illustrated in Figure 1 is illustrated in Figure 2 in which 1 is a complete focus coil and screening assembly of the kind shown in Figure 1, while 8 and 9 are brackets in each of which is an apertur'e having partially circular edges I0 which bear upon the flange I l of the coil 1 so as to constitute in effect a spherical mounting for the latter. Obviously this arrangement may be modified without departing from the feature that a comparatively thin edged circular disc to which the focus coil is attached is retained in position between two members having apertures therein, the edges of the apertures upon which the said disc bears being formed as circular arcs.

I claim:

1. Focusing means for use with an electron discharge -device comprising an electromagnetic coil and ferromagnetic shielding means surrounding said coil, said shielding means comprising a plurality of pairs of members, each of said members comprising a cylindrical portion, an annular portion extending inwardly from one end of said cylindrical portion and a flange extending outwardly from the other end of said cylindrical portion, said members being assembled with their flanges abutting and the lengths of the said cylindrical portions varying from one pair of members to another' so that when the said flanges are placed in contact a space remains between the annular portions of one pair and those of the next larger pair. p 2. Focusing means for use with an electron discharge device and ferromagnetic shielding means surrounding said coil, said shielding means comprising two pairs of members, each of said members comprising a cylindrical portion, having an external ange at one end and an internal flange at the other end, one of said pairs of members being of such size as to i'it closely round said coil with the said external flanges in contact and the other of said pairs of members being arranged to fit over said first pair so that when the external anges of said second pair abut the external flanges of said first pair, a space 'remains between the internal flanges of said first and second pairs.

3. The combination with focusing means according to claim 1 of supporting means comprising two members each haxing an aperture with two arcuate edges, said arcuate edges gripping the flanges on said focusing means.

DENIS VICTOR RDJGEWAY. 

